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Gravity runs the moons too — and your weight

Grades 7–8 · 45 minutes · Explain the mechanism. The driving question: Why does everything out there orbit — and why don't the planets fall into the sun? Every step below is a link that opens the exact view — one link puts the projector and every student screen on the same sky. Part of the lesson plans by topic and grade.

Before the lesson — what this assumes

Assumes: closer-to-the-sun means faster — two minutes of the inner planets on Play re-establishes it.
Assumes: the difference between mass (how much stuff) and weight (how hard gravity pulls on it).
Pre-work: skim Jupiter's page and write down its surface gravity — today the class finds out where that number comes from.

The plan — every step carries its minutes

Warm-up · 5 minAsk: what would you weigh on Jupiter? Guesses on the board — most will say “ten times as much”, and the real answer is the surprise the lesson is built around.

Task 1 · 10 minJupiter's four big moons, a month on the slider, Play. From the read-out, record each moon's orbital period: Io, Europa, Ganymede, Callisto. State the pattern — inner is faster — and say what it is: the planets' rule, running again around a planet. One law, every scale.

Task 2 · 6 minSaturn's system: does the rule repeat? Have them check two moons' periods against their distances before you say anything.

Task 3 · 12 minThe weight question, with real numbers: the planet pages derive surface gravity from mass and radius — Mars, Jupiter, Saturn. Each student computes their own weight on all three. Then the trap springs: Saturn is 95 Earth masses, and you would weigh about the same as at home. Why? (It is huge but puffy — gravity depends on mass AND radius, g = GM/r².)

Task 4 · 6 minThe honesty check: the moon views state that positions along the orbits are illustrative. Ask WHY sizes, distances and periods can be right while positions are not (a position needs an epoch — a measured starting point — and an orbit does not). What would we need to fix it?

Wrap-up · 6 minOne diagram per student: sun → planets, planet → moons, the same arrow labelled the same word. And the weight table, with Saturn circled.

The minutes are there for the teacher's pacing — and so that any single step can be handed to a student as their own five-minute lesson. See students teach the class.

What they should find — the teacher's key

Io ≈ 1.8 d, Europa ≈ 3.6 d, Ganymede ≈ 7.2 d, Callisto ≈ 16.7 d — and the doubling pattern in the first three is a resonance worth mentioning.
Surface gravity: Mars ≈ 0.38 g, Jupiter ≈ 2.5 g, Saturn ≈ 1.07 g. The Saturn result is the discussion.
g = GM/r² does all of it; the site derives those figures the same way, so the class's numbers match the pages'.

Standards: MS-ESS1-2 (gravity's role in the solar system); MS-ESS1-3.

What the picture fakes: On every system view the planet dots are drawn far larger than scale, and where each moon sits along its orbit is illustrative — sizes, distances, speeds and directions are real, positions are not solved, and the pages say so.

Questions to chase on your own

For the student who wants more — each answerable with the tools, no teacher required:

Compute your weight on Uranus and Neptune. Which surprises you, and why?
Io, Europa and Ganymede orbit in a locked 1:2:4 rhythm. Find the name for that on Jupiter's page and explain it in one sentence of your own.

Questions the curious actually ask

Real questions, mostly from real kids — the kind that sound simple and open trapdoors. Worth raising in class before someone raises them for you:

If gravity pulls everything, why doesn't the moon fall on us?
Here's the secret: it is falling — right now, continuously — and missing. The moon moves sideways so fast that by the time it has fallen toward Earth, the Earth's surface has curved away beneath it. An orbit is a fall that never lands. Newton figured this out by imagining a cannon on a mountain firing faster and faster until the cannonball fell all the way around the world — and that thought experiment is every satellite, the space station, and the moon.

Why don't the planets ever crash into each other?
Mostly they can't — each is locked in its own lane by its own speed, and the lanes are separated by distances that make the planets themselves like grains of sand miles apart. But here's the part worth knowing: early on, they did crash. The leading theory for where the moon came from is a Mars-sized world hitting the young Earth. The quiet, orderly system in the simulator is the survivor of a demolition derby — the orbits you see are the ones that lasted.

How does the sun keep burning? There's no air in space!
Because it isn't burning — fire needs air, and the sun would have burned out in a few thousand years if it were a bonfire. The sun is crushing: its own gravity squeezes its core so hard that hydrogen atoms fuse together, and fusion releases millions of times more energy than fire. It has run this way for 4.6 billion years and is about halfway through its fuel. Nothing is on fire. Something much stranger is happening.

Go further — beyond this site

NASA: Jupiter's Moons — the reference list — 90+ known, four that matter today.
PhET: Gravity and Orbits — turn gravity OFF and watch what the moons do — the counterfactual the simulator can't show.

Same question, other grades

Each grade band re-asks this topic's question one level deeper — observe it, describe the pattern, measure it, explain the mechanism, quantify it and question the model. This page is the explain the mechanism rung.

Grades K–2 Grades 3–4 Grades 5–6 High school & up All lesson plans

Teachers: make this lesson better

You are the one standing in front of the class, so you will see what we cannot: a task that runs long, a question that lands better another way, a grade level pitched wrong, a topic we should build next. Tell us — improvements go into the page, and if we use yours, your class gets the credit on it, the same promise the classroom request form makes.

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